Patent Document 1 discloses a multi-protection valve including an inlet port connected to a pneumatic device as a compressed air source, a parking brake circuit outlet port (referred to as “spring brake circuit outlet port” in Patent Document 1) connected to a parking brake circuit (referred to as “spring brake circuit” in Patent Document 1), and a connection passage connecting the inlet port and the parking brake circuit outlet port. The connection passage includes a check valve for preventing air from flowing from the inlet port into the parking brake circuit outlet port and a throttle for regulating the flow rate of exhaust air flowing in from the parking brake circuit through the parking brake circuit outlet port, the check valve and the throttle being disposed in series.
The connection passage allows air in the parking brake circuit to be discharged to the outside when air leakage occurs in a first main brake circuit or a second main brake circuit suffers to make the first main brake circuit or the second main brake circuit inoperable. When the air in the parking brake circuit is discharged to the outside, the parking brake cannot be disengaged, that is, the engagement of the parking brake can be maintained, which prevents unprepared starting of a vehicle during a period from engine start until the first main brake circuit or the second main brake circuit becomes operable. The configuration also prevents the residual air pressure in the parking brake circuit from exceeding the residual air pressure in the first main brake circuit or the second main brake circuit.    Patent Document 1: JP-A-Hei 10-100888